Introduction

Metallic NPs and nanostructures perform a very effective role acting as optical antennas, as has been introduced in previous chapters. Together with their functionality in transferring electromagnetic energy to the far-field in a directional manner [141–143], they can also localize this energy from the far-field into the near-field, an effect of utmost importance in field-enhanced spectroscopies, as we will review in this chapter.

Field enhancement

Optical antennas are able to localize the electromagnetic field by means of excitation of LSPRs in the metal. These matter excitations are associated with oscillations of the surface charge density at the interface between the metal forming the nanostructure and the outer medium. Different metallic nanostructures are arranged in a variety of designs, sometimes mimicking and reproducing previous ones in RF. Depending on the particular role that an optical antenna needs to fulfill, it is possible to find linear antennas for dipolar emission [144], λ4 antennas for omni-directional emission [145], Yagi-Uda antennas for directional emission [81, 143, 146], patch antennas [147] or even parabolic-like nanocups that bend light similarly to parabolic antennas [148]. All these emission properties have their origin in a particular excitation of electromagnetic modes in the nanostructure.

For spectroscopic applications, it is important to consider not only the emission properties of the antenna, but also the localization and strength of the local fields.